1. Field of the Invention
The present invention relates to a liquid developer, and more particularly, to a high-resolution liquid developer and a method for preparing the same.
2. Description of the Related Art
Printers can be classified into laser printers and inkjet printers. Laser printers form a latent image on a photoconductor using a laser and a toner image from the latent image by applying toner to the photoconductor using a potential difference, and transfer the toner image to paper to obtain a desired image. Inkjet printers are known to be unsuitable for a high throughput task directed by a plurality of computers connected via a network because the printing speed of the inkjet printer is slow.
Most laser printers in current use are black-and-white dry-type printers. In dry-type printers, a solid toner is caused to frictionally charge the air to migrate onto a latent image formed on an organic photoconductor (OPC). However, it is known that the use of solid toner particles raises environmental concerns due to toner scatter, and image quality is limited. In wet-type printers, a toner dispersion is used. The solvent of the toner dispersion serves as a carrier for toner particles migrating to the latent image on the OPC, and thus the development process is less affected by ambient factors, such as temperature and humidity. Also, the environmental concerns due to toner scatter can be overcome by the use of liquid toners. Toner particles for use in the wet-type printer can be controlled to be much smaller than those for dry-type printers (having a particle size greater than 6 μm), resulting in quality images.
There are two types of liquid toners. The first is a resin-based common toner, and the second is an organosol toner using self-stable organosols. Self-stable organosols refer to colloidal particles (0.1-1 micron diameter) synthesized by non-aqueous dispersion polymerization in a low-dielectric hydrocarbon solvent. These organosol particles enhance the dispersion properties of colorants to be resistant to sedimentation and aggregation. In the most commonly used non-aqueous dispersion polymerization method, one or more ethylenically-unsaturated (typically acrylic) monomers, soluble in a hydrocarbon medium, are polymerized in the presence of a preformed amphipathic polymer (stabilizer). The stabilizer amphipathic polymer includes two distinct repeat units, one essentially insoluble in the hydrocarbon medium, the other freely soluble. When the polymerization continues to proceed beyond a critical molecular weight, the solubility limit is exceeded and the polymer precipitates, forming a core particle. The amphipathic polymer then either adsorbs onto or covalently bonds to the core, which continues to grow until the monomer is depleted. The amphiphathic polymer “shell” acts to stabilize the growing core particles with respect to aggregation. The resulting core/shell polymer particles are spherical, and have a diameter in the range of 0.1-0.5 microns.
The resolution of laser printers is known to be closely associated with toner particle diameter. Various attempts have been made to improve the resolution of printer toners.
In a method disclosed in U.S. Pat. No. 5,407,771, as shown in FIG. 1, initially a pigment, a dispersant, and a resin are mixed with a solvent and stirred at a temperature of 65-100° C. until the components are thoroughly mixed. Once the components have been thoroughly mixed, the temperature of the reactor is slowly dropped to precipitate solid resin from the melted resin. The isolation of the solid resin results in solid ink, and the solid ink is ground using a cutter or grinder for 75 hours with an addition of a solvent, resulting in a liquid developer. A charging control agent (CCA) to provide the liquid toner with electrostatic properties is added after or during the wet milling. This conventional process is complicated and costly because it involves high-temperature mixing and long-time milling. Also, the initial particle size or the dispersion properties of the pigment used for the formation of toner particles considerably affects the final toner particle diameter. Therefore, it is difficult to obtain toner particles of a uniform size.
A liquid developer preparation method using a cosolvent is disclosed in U.S. Pat. No. 5,876,896. In this method, as shown in FIG. 2, a pigment, a resin, a cosolvent, a solvent, and a dispersant are placed in a reactor and are stirred while maintaining the reactor at a temperature at which the resin is soluble in the cosolvent and solvent. Next, the temperature is slowly dropped to precipitate the resin shelling the pigment, so that toner particles are obtained. Next, the cosolvent is replaced by a hydrocarbon solvent, followed by an addition of a CCA to complete the preparation of a liquid toner. According to this method, the final toner particle diameter and its distribution are controlled by adjusting a mixing ratio of two liquids constituting the cosolvent as a solubility parameter (SP value) of the carrier liquid. However, the use of cosolvent, usually, toluene, is hazardous for the human body and raises environmental problems.
U.S. Pat. No. 5,652,282 discloses a liquid developer using organosols. However, since the liquid developer contains toner particles as large as 3.1-3.5 μm, a defective flow pattern appears, as shown in FIG. 3, after development onto the OPC by a developing roller. As a result, the final image is also indistinct and has very irregular thicknesses, as is apparent from FIG. 4.